Direct Carbothermal Reduction Research Articles

Mechanism and kinetics for the carbothermal reduction of arsenic-alkali mixed salt produced from treatment of arsenic-alkali residue

Arsenic-alkali mixed salt (AAS) is a hazardous by-product of the treatment of arsenic-alkali residue (AAR). Although direct carbothermal reduction can enable the harmless and resourceful disposal of AAS, the specific reaction pathway and mechanism of the reduction process remain unclear. To gain further insight into the reaction process, the non-isothermal kinetics and reaction mechanism of AAS carbothermal reduction were investigated. The results indicate that AAS reduction involves three pathways: (1) direct reduction of Na2As4O11 to As(g), (2) reduction of Na2As4O11 to As2O3 and subsequent reduction to As(g), and (3) direct reduction of Na3AsO4 to As(g). Calculations using the Flynn-Wall-Ozawa (FWO) and integral master-plot methods indicate that the kinetic model conforms to the two-dimensional diffusion function, with an average activation energy of 164.30 kJ/mol and a pre-exponential factor of 1.764 × 106 1/s. These results are significant for further in-depth research into the carbothermal reduction of AAS.

Optimal Process Parameters for Direct Carbothermal Reduction of Vanadium–Titanium Magnetite in a Rotary Kiln

Direct reduction in rotary kiln plays a crucial role in the reduction and smelting of vanadium–titanium magnetite. The effect of the process parameters on the metallization rate and reaction mechanism during the reduction process in a rotary kiln is investigated. The results indicate that the optimal process parameters for direct reduction of vanadium–titanium magnetite in a rotary kiln are as follows: a reduction temperature of 1150 °C, reduction time of 4 h, carbon ratio 1.2, particle size of vanadium–titanium magnetite as 8–12 mm, and particle size of pulverized coal of <8 mm. During the direct reduction of vanadium–titanium magnetite, favorable conditions such as absence of smelting phenomenon on the surface of the reduction products, high metallization rate, and high quality of direct reduction iron are achieved. Through direct reduction, the transformation process of iron oxide in vanadium–titanium magnetite is Fe3O4 → iron oxide → Fe, and the transformation process of titanium–iron oxide is Fe9TiO15 → Fe2.75Ti0.25O4 → Fe2.5Ti0.5O4 → iron titanate → Fe. In addition, vanadium and ferrochrome oxides undergo reduction to form oxides or carbides.

Magnetic‐Dielectric Complementary Fe‐Co‐Ni Alloy/Carbon Composites for High‐Attenuation C‐Band Microwave Absorption via Carbothermal Reduction of Solid‐Solution Precursor

AbstractFerromagnetic alloys/carbon composites with excellent electrical and magnetic properties are highly desirable as electromagnetic wave absorption materials, but achieving high‐attenuation performance in C‐band (4–8 GHz) remains a challenge. Herein, a direct carbothermal reduction of organic gluconate solid‐solution precursor method is developed to synthesize ferromagnetic Fe‐Co‐Ni alloy/carbon composites, which realize high‐attenuation electromagnetic wave absorption in C‐band. By virtue of Fe, Co, and Ni elements homogeneously dispersing at the molecular level in the solid‐solution precursor with the regulated mole ratio, serial FeCo2Ni/C, Co7Fe3/C, FeNi3/C, and Co3Ni/C composites can be deliberately prepared. First‐principles calculations and off‐axis electron holograms can clearly unravel that these Fe‐Co‐Ni alloys could perform as excellent dielectric‐magnetic complementary loss units to trigger synergistic electronic dipole polarization oscillation, magnetic moment resonance, and magnetic coupling interaction. Meanwhile, the rich alloy–carbon interfaces and conductive carbon skeleton can facilitate delightful interfacial polarization and conductive loss. Combining these positive electromagnetic energy dissipation characteristics, FeCo2Ni/C with strengthened dipole polarization oscillation and outstanding impedance matching realizes an extremely high‐attenuation absorption performance with a minimum reflection loss of −82.2 dB at 5.21 GHz. This work provides a feasible and flexible insight into producing magnetic‐dielectric complementary Fe‐Co‐Ni alloys/carbon composites with various alloy compositions as excellent electromagnetic wave absorbers.

Microwave-assisted carbothermal reduction of rice hull ash to biogenic silicon

Rice hull is an agricultural waste disposed of globally in large quantities. However, it is an attractive biomass resource, containing at least 15–20 wt% amorphous silica. Herein, biogenic Si production was attempted by direct carbothermal reduction of rice hull ash using a microwave heating source. Si was prepared from rice hull ash with a C-to-SiO2 ratio (in mole ratio) of 1.0 at 1564 °C after 15 min of 1500 W microwave irradiation. Although it is still difficult to evaluate the yield, the possibility of producing metallic Si at lower temperatures than the traditional method (>2000 °C) has been demonstrated, and it is hoped that this method will be applied in the future. We expect that these results can contribute to creating a new process that enables low-cost, energy-saving, and high-efficiency production of Si for solar cells and semiconductors.

Resource-saving production of Fe-based amorphous alloys from carbothermal reduction of high-phosphorus oolitic iron ore

Energy-saving and resource-friendly production of Fe-based soft-magnetic materials is of paramount importance for the development of green economy. In this study, an innovative process for fabricating Fe-based amorphous alloys from high-phosphorus oolitic iron ore was developed with the combination of ironmaking and melt-spinning techniques. By using the developed process, iron and phosphorus in high-phosphorus oolitic iron ore can be simultaneously utilized. During the direct carbothermal reduction process of high-phosphorus oolitic iron ore, oolitic hematite and apatite were first reduced to form liquid iron phase and absorb P2 gas, resulting in the enrichment of P element in iron phase. Fe-P alloys with different P and C content can be easily obtained by tailoring the C/O ratio. The reduced Fe-P alloys with high εFe of 96.4% and εP of 70.7% are ideal raw materials for preparing Fe-based soft-magnetic alloys. Fe80P10C9Si1 amorphous alloy was successfully obtained after composition adjustment. The high AFA of rod sample with diameter up to 1 mm and good magnetic softness of the ribbon samples including high Bs of 1.54 T, low Hc of 3.6 A/m, high μe of 30 × 103 with excellent frequency stability prove the effectiveness of the designed process. These findings are of great significance to lay the foundation for the recycling of resources and preparation of high performance soft-magnetic alloys from iron ores.

Forming thermodynamics, structure, and electrical conductivity of TiCxOy compounds fabricated through the carbothermal reduction process

In this study, TiCxOy was fabricated using TiO2 as a Ti source through a carbothermic reduction process in an Ar atmosphere. Six reaction types were considered during the fabrication process. Based on thermodynamic analysis, the reduction of TiO2 to TiCxOy could only be realized through the first or second reaction type, involving the direct carbothermal reduction and simultaneous CO production. The third and fourth reaction types, which also involved the direct carbothermal reduction with simultaneous CO2 release, could only reduce TiO2 to TinO2n−1 (n≥2), not to TiCxOy. The fifth and sixth reaction types could not occur. The C content in the raw material and the sintering temperature had significant effects on the structures and electrical conductivities of the samples. The TiCxOy phase began to form in the samples sintered at 1673 K with a C/TiO2 mole ratio of 2 and transformed into a single TiCxOy phase completely at 1823 K according to X-ray diffraction results. The TiCxOy samples crystallized with a layered formation structure had a coral shape, and Ti, C, and O were distributed evenly in the samples according to scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy analyses. High-resolution X-ray photoelectron spectroscopy results showed that Ti existed in the samples with II, III, and IV valence states. The electrical resistivity decreased with the increase in the C content and sintering temperature. The mean resistivity and conductivity of the fabricated samples with a single TiCxOy phase were determined to be 5.04×10−5 Ω m and 1.99×104 S m−1, respectively, through four-probe measurements. Theoretically, TiCxOy proved to be a good electrical conductor through the analysis of the band structure and the density of states based on density functional theory calculations, which agreed with the experimental results.

SIMULATION OF THE DIRECT CARBOTHERMAL REDUCTION OF SiO2 FOR Si METAL USING MATLAB® AND ASPEN-HYSYS®

Silicon metal is one of the most important metals produced from silica by carbothermic reduction in the electric arc furnace which works at high temperatures between 1000 to 3000 oC. The objective of this work is to simulate the direct carbothermal reduction of SiO2 for Si metal by using a thermodynamic calculation of Gibb’s free energy minimization with MATLAB® and Aspen HYSYS®. The comparison of simulation using MATLAB® with reference [10] revealed close results. By increasing the mole ratio up to 10 increases the Si formation and increases the ratio more than 10 mole ratio had very bad results,and not economically viable. Here we did comparison between simulation results using MATLAB® with real assumption and ASPEN HYSYS®.

Production of Si3N4 porous beads via carbothermal reduction and nitridation technique

ABSTRACT Si3N4 porous beads were produced from direct carbothermal reduction and nitridation of spheres composed of carbon+SiO2 mixtures. A simple one–step sol–gel templating technique was used to prepare carbon+SiO2 (molar ratio of carbon/SiO2 was set as 4) containing spheres with a diameter of 2 mm. A natural and abundant biopolymer, alginate, is used as a sacrificial template to produce porous Si3N4 ceramic beads. Carbothermal reduction and nitridation process was conducted at two different temperatures (1500 and 1550°C) and atmospheres (pure nitrogen and 5% hydrogen in nitrogen) to observe the effect of process parameters on the yield of Si3N4. Porous beads with a smooth shape and without any deformation were successfully obtained after the carbothermal reduction and nitridation process. X–ray powder diffraction studies showed that the use of H2 increased the amount of Si3N4 formed by accelerating the reduction and nitridation reactions. Microstructural investigations revealed oxide addition changed grain morphology from fiber–like to short, angular geometry. This microstructural development showed that the addition of oxide powder increased the amount of liquid phase formed during the heat treatment process and changed carbothermal reduction and nitridation mechanism from vapor–solid to vapor–liquid solid.

A novel approach for preparation of dense TiC–SiC nanocomposites by sol–gel infiltration and spark plasma sintering

Abstract Dense TiC–SiC nanocomposite ceramics were prepared by infiltration of porous TiC scaffolds with a Si O C sol, followed by spark plasma sintering (SPS). The porous nano TiC scaffold was first synthesized by direct carbothermal reduction of a monolithic Ti O C precursor obtained from a controlled sol–gel process. The TiC scaffold was infiltrated with a Si O C sol and then the sample was aged in a container for 48 h at 80 °C to convert the sol into gel. After this, the sample was heated at 550 °C to remove the organic components and then 1350 °C to convert the Si O C gel to SiC by carbothermal reduction reaction. The cycle of the infiltration and carbothermal reduction was repeated several times to obtain relatively dense TiC–SiC composite samples. Dense TiC–SiC composite with a uniform nano-sized grain microstructure was obtained by spark plasma sintering at 1800 °C for 5 min under 40 MPa uniaxial pressure. Compared with conventional powder mixing methods, the sol–gel infiltration approach has shown distinct advantages of achieving dense TiC–SiC composites with uniform nano-sized grain structures.

Alternative Al production methods

The production of Al from its ores at present relies on the Bayer (alumina production) and the Hall–Heroult (Al production) process. The cost associated with alumina production and apparent disadvantages of the Hall–Heroult process have led to intensive research to find alternative routes for Al production. The direct carbothermal reduction process has been thoroughly investigated as an alternative technique. Another alternative includes the indirect carbothermal reduction route where alumina (or aluminous ores) is first reduced to intermediate Al compounds before reduced further to Al. The present study reviews and provides systematic thermodynamic analyses of alternative Al production routes. In this paper (Part 1), a comprehensive review of alternative Al production techniques focusing on the indirect carbothermal reduction routes is presented. These include carbochlorination, carbonitridation and carbosulphidation routes for the formation of intermediate Al compounds, followed by various Al extraction processes.

Synthesis of Mesoporous Carbon‐Bonded TiC/SiC Composites by Direct Carbothermal Reduction of Sol–Gel Derived Monolithic Precursor

Mesosporous carbon‐bonded titanium‐carbide/silicon‐carbide (C‐TiC/SiC) ceramics with a high specific surface area (221 m2/g), ultra fine grains (10–50 nm), and high crystallinity were synthesized from direct carbothermal reduction (1100°–1450°C) of monolithic Ti–Si–O–C precursors made from controlled sol–gel process. The nano‐sized carbide grains are bonded into bulk materials by poly(furfuryl alcohol) (PFA) derived nanocrystalline carbon framework by “bridge,” “entanglement,” and “adhesive” effects. Graphenes were found to grow on {111} planes of TiC grains but this was not observed for SiC. The bonding of graphitic carbon layers on carbide grains support the nanostructure and also result in the desired combination of functional and mechanical properties.

A simulation study on the direct carbothermal reduction of SiO 2 for Si metal

The direct carbothermal reduction of SiO 2 to Si metal was simulated as a function of mole ratio of SiO 2, SiC and C in the temperature range of 0–3000 °C by using a thermodynamic calculation of Gibb’s free-energy minimization. In the SiO 2–C system, the silicon metal was formed from the 1250 °C and completed at 1500–2000 °C with the excess amount of C. By the addition of SiC in SiO 2–C carbothermal reduction system, complete conversion of SiO 2 to Si could be promoted through the lowering the reaction temperature and the excess carbon. Methane gas was efficient reducing agent of SiO 2 to form Si metal at around 2000 °C, while H 2 and CO could not reduce SiO 2 to Si at any conditions. The simulated results were consistent with the experimental works carried out in the arc furnace.

Solar Production of Aluminum by Direct Reduction: Preliminary Results for Two Processes

The production of aluminum or silicon by reduction of their oxides with carbon is a technical challenge. The temperature required, in the range 2100–2300°C, is too high for practical process heat addition from a combustion source alone. When an electrothermal process is used, only about a third of the energy contained in the fuel used to generate electricity enters the process. Thus, for materials produced electrolytically or in an electric furnace, the energy cost dominates the cost of the final product. By contrast, highly-concentrated solar energy is capable of supplying large amounts of process heat at very high temperatures, and may have real advantages for metals reduction processes. An arc introduces too much energy to the reaction zone. In the case of aluminum, the metal floats and it short circuits the arc. Ideally, the heat would enter at the bottom or side of a reactor, which could be achieved with solar process heat. Among industries, the primary aluminum industry is a major consumer of electricity. It uses about 10 percent of the electricity generated globally for industrial purposes, and about half comes from coal-fired generation stations. This represents about 5 percent of the electricity generated for all sectors. A solar-thermal process would drastically reduce the emission of climate-altering gases, reduce the reliance on electricity, and might be a critical factor in making a direct thermal route from the ore to metal possible. Two industrially-developed processes appear to be attractive candidates for a solar process. Preliminary tests have been performed using a black-body cavity receiver placed at the focus of the Paul Scherrer Institute’s 70 kW tracking parabolic concentrator, and though the experiment had to be ended earlier than planned, a small amount of 61/37 weight percent Al/Si alloy was formed, and the partially reacted pellets showed conversion to Al4C3 and SiC. Further qualitative tests have been performed using the facilities at Odeillo in a 2 kW solar furnace, where the onset of production of both aluminum by direct carbothermal reduction, and Al-Si alloy via carbothermal reduction of a mixture of alumina, silica and carbon could be directly observed.

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β‐SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si3N4 could be completely converted to SiC with molar ratio Si3N4:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.